1. Field of the Invention
The present invention relates to radiation-crosslinkable mixtures which contain soluble polymeric precursors of heterocyclic polymers stable at high temperatures, copolymerizable ethylenically unsaturated compounds, photoinitiators, solvents and, if required, further assistants and additives, and their use for the production of relief structures which are stable at high temperatures, heterocyclic polymers stable at high temperatures being in particular polyimides, polyisoindoloquinazolinediones and polyaroylenebenzimidazoles whose precursors still carry free carboxyl groups.
2. Description of Prior Art
It is known that relief structures which are stable at high temperatures can be produced, in particular for the production of electronic circuits, by first applying soluble, radiation-sensitive, preferably UV-sensitive precursors A from solution onto a substrate and drying them under mild conditions, then forming a crosslinked intermediate B under a mask using radiation of a suitable wavelength then washing out the unradiated, i.e. unexposed, parts, which still contain the precursor A, with suitable solvent mixtures and then converting the resulting relief structures into the final functional structure of stage C with the use of high temperatures (heating step).
For the production of such relief structures which are used, for example, as interlayer dielectrics or as passivation layers for microchips, the abovementioned heterocyclic or aromatic/heterocyclic polymers are preferably used as substances of functional stage C, owing to the high thermal and mechanical loads during the preparation.
The soluble precursors A for such polymers C are generally compounds having a polyamide parent structure, i.e. those in which another group, as a rule a carboxyl or ester group, is ortho or peri to the amido group and undergoes a condensation reaction with the amido group during the conversion in stage C with the use of elevated temperatures.
A method for rendering polymeric, polyamide parent structures of stage A radiation-crosslinkable is the introduction of unsaturated side groups, as illustrated for a polyamidoester: ##STR1##
The introduction of unsaturated groups into polymeric precursors A is described in, for example, DE-A-2 473 397, DE-A-2 437 348, DE-A-2 437 413,DE-A-2 437 369, DE-A-2 919 840, DE-A-2 919 841, DE-A-2 933 826 and DE-A-2 308 830.
In general, the following three methods are used for synthesizing photosensitive precursors A according to the prior art:
a) The introduction of photosensitive groups, as a rule ethylenically unsaturated groups, into the monomer building blocks and subsequent synthesis of the higher molecular weight precursors. PA1 b) Another method for introducing photosensitive unsaturated groups comprises first synthesizing polymeric or oligomeric products without photosensitive groups and then introducing photosensitive groups into these products. PA1 c) In order to avoid these problems, other patents propose likewise first synthesizing non-photosensitive polyamic acids as in method b) and subsequently rendering them photosensitive in a very mild manner by simply adding unsaturated amines which form salts with polyamic acids (cf. for example DE-A-2 914 619). PA1 (I) at least one carboxyl-containing polymeric precursor of heterocyclic polymers stable at high temperatures, which precursor is soluble in polar organic solvents, PA1 (II) at least one copolymerizable ethylenically unsaturated ternary sulfonium salt, PA1 (III) at least one photoinitiator or photoinitiator system and PA1 (IV) at least one polar aprotic organic solvent. PA1 R.sup.1 =aliphatic, cycloaliphatic, araliphatic, aromatic or heterocyclic radical which preferably contains a polymerizable multiple bond. PA1 a) Leveling agents, which are used for obtaining smooth, pit-free surfaces, PA1 b) Sensitizers, which improve the photosensitivity by extending the absorption range of the photoinitiator used, by effective transmission of radiation energy to the photoinitiator, e.g. anthraquinone, 2-methylanthraquinone, 2-ethylanthraquinone, benzoquinone, 1,2-naphthoquinone, 1,4-naphthoquinone, 1,2-benzanthraquinone, benzophenone, 4,4'-dimethylbenzophenone, Michler's ketone, 2-nitrofluorene, 5-nitroacetonaphthene, 4-nitro-1-naphthylamine, anthrone, 1,9-benzanthrone, dibenzalacetone,.4,4'-bis(diethylamino)benzophenone, acridine, cyanoacridine, nitropyrene, 1,8-dinitropyrene, pyrene-1,6-quinone, 2-bromobenzanthraquinone, 9-fluorenone, 2-chloro-1,8-phthaloylnaphthalene, etc., PA1 c) leuco dyes, e.g. leuco crystal violet, and PA1 d) adhesion promoters, e.g. 3-azidopropyltriethoxysilane. PA1 Viscosity/solids content: 11,000 mPa.s/30% PA1 Acid number: 53.3 (theory 53.5) PA1 Viscosity/solids content: 4,000 mPa.s/30% PA1 Acid number: 53.4 (theory: 53.5) PA1 Viscosity/solids content: 1,600 mPa.s/30% PA1 Acid number: 53.5 (theory: 53.5) PA1 Viscosity/solids content: 630 mPa.s/30% PA1 Acid number: 53.6 (theory: 53.5) PA1 Viscosity/solids content: 14,000 mPa.s/30% PA1 Acid number: 45.6 (theory: 45.8) PA1 Viscosity/solids content: 15,000 mPa.s/30% PA1 Acid number: 46.3 (theory: 46.6)
A typical example of this method is described in DE-A-2 437 397, i.e. the addition reaction of allyl alcohol with pyromellitic anhydride, the reaction of the resulting half-ester with thionyl chloride to give the acyl chloride and conversion of the latter into the polymers by reaction with aromatic diamines. PA2 A typical example of this method is described in, for example, DE-A-2 933 826. Here, pyromellitic anhydride and diaminodiphenyl oxide in dimethylacetamide as a solvent are subjected to an addition reaction to give a polyamide which contains carboxyl groups. Double bonds are then introduced into these carboxyl groups by means of glycidyl methacrylate, resulting in the precursors A which are crosslinkable in the presence of photoinitiators under UV light. PA2 The principal disadvantages of the prior art method a) are the expensive reaction sequence and the difficulties in synthesizing the polymeric precursors A, which arise from the unsaturated nature of the starting materials and of the end products. PA2 It is also difficult subsequently to introduce double bonds into polymeric products by method b). Here, for example, only relatively low temperatures may be used, resulting in long reaction times and poor conversions. Furthermore, polymerization inhibitors must nevertheless be added in some cases during the preparation and subsequently removed in expensive purification steps in order to achieve high photosensitivity during use. PA2 Furthermore, a number of the abovementioned patents therefore relate not to the principle but to optimization of the methods for the preparation of the polymeric, photosensitive precursors. Nevertheless, their preparation remains critical. PA2 The main problems in this method are the poor solubility of the polymer salts and their high viscosity, so that only solutions having low solids contents can be prepared. Finally, the precursors prepared according to c) have, at room temperature, a shelf life which is insufficient for processing. PA2 A further fundamental problem relates to the heating step of the precursors A prepared according to a) or b). Since the unsaturated groups are covalently bonded to the precursors of the heterocyclic polymers, high temperatures must be used for relatively long times in order to achieve quantitative cyclization. During this procedure, not only do the relief structures produced become black but the desired mechanical properties, such as flexural strength and buckling resistance, are not achieved. As a result of the trend in microelectronics toward increasingly large and increasingly highly integrated chips, it is precisely these properties which are becoming more and more important for ensuring the desired reliability of the components produced and for minimizing the defect rate in production.
Further prior art publications which describe synthesis routes starting from unsaturated monomer building blocks are, for example, DE-A-2 437 369, DE-A-2 437 413, DE-A-2 919 840, DE-A-2 919 841, DE-A-3 411 660, DE-A-3 411 697, DE-A-3 411 706, DE-A-3 411 714, U.S. Pat. No. 4,551,522 and U.S. Pat. No. 4,558,117.